Your pilot volume looks a bit big, it's even worse that it is using PVC fittings as they seem to have more volume than same sized metal fittings.
Just try to keep it as small as you can, and if you can't make it any smaller, up the pilot valve.

You'll notice not every fitting will fit together perfectly, so if a fitting can only fit halfway in to another fitting, this creates more volume. Yes, the pilot volume is quite essential to making a good valve. If you have a big enough pilot valve, it doesn't matter so much. Just remember the smaller pilot volume you have, the faster your valve will open.

If your pilot volume is too big, your pilot valve too small and your piston isn't sealed with o-rings, your piston might not even open at all.
Just something to keep in mind.

The pilot volume is between the piston and the pilot valve. The pilot valve in this case is that 1/2" ball valve you have. Anything on the left side of that ball valve doesn't matter as long as it can allow air to escape in to the atmosphere.

Just remember that the faster you can get the air pressure in the pilot volume out in to the atmosphere, the faster your valve will open. Ways to do this are; you need to minimize the pilot volume and maximize the pilot valve. Or you can do one or the other, big pilot valve means you can have a big pilot volume. Small piot volume means you can have a smaller pilot valve.

Just build your cannon how you think is right, and then if you have troubles with the piston later we can sort that out.

i've been thinking... i cant think of any ways to get less pilot volume... because i have to pump it up through a schrader that has to be to the left of the piston so it can push it into the barrel, right? so how am i supposed to lower the pilot volume? i may use metal fittings, which may help... but will that be enough?